Method and apparatus for collision avoidance in sensor network

ABSTRACT

Disclosed is a method and apparatus for collision avoidance in a sensor network. The method includes transmitting data by a transmitter; comparing, by the transmitter, a remaining time between the data transmission and a backoff period with a turnaround time taken for the transmitter to switch from a transmit (Tx) mode to a receive (Rx) mode; selectively transmitting, by the transmitter, a busy signal indicating that a channel is in use, according to the comparison result; and receiving, by the transmitter, an acknowledgement (ACK) signal from a receiver after the turnaround time has elapsed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Korean PatentApplication No. 10-2011-0021841 filed on Mar. 11, 2011, in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein by reference in its entirety.

BACKGROUND

1. Field

The present invention relates to a method and apparatus for collisionavoidance in a sensor network, and more particularly to a method andapparatus for collision avoidance when nodes use communication channelsto exchange data therebetween in a sensor network based on a slottedIEEE 802.15.4 standard, and a recording medium recording the method.

2. Description of the Related Art

A sensor network, which is one of core technologies for a ubiquitouscommunication system, has been used in a variety of industrial andcustomer applications, such as distribution automation, security,industrial process automation and control, smart home and U-health care.The sensor network typically employs IEEE 802.15.4 which is a standardthat specifies the physical layer and the link layer for low-ratewireless personal area networks (LR-WPANs). It is the basis for theZigBee which has been employed in a variety of applications.

The sensor network is built of sensor nodes. The sensor node has to beas low in power consumption and manufacturing cost as possible. Lowenergy consumption is one major challenge in the sensor network.Distribution of sensor nodes equipped with mechanism to reduce energyconsumption in the sensor network will help increase the lifetime of thesensor network. Such mechanism to reduce energy consumption may beapplied to any type of ubiquitous communication system as well as thesensor network.

SUMMARY

The present invention relates to a method and apparatus for collisionavoidance in a sensor network, which is capable of preventing sensornodes from performing unnecessary clear channel assessment to check if acommunication channel is busy since it takes time for communicationhardware of sensor nodes in the sensor network to switch from transmit(Tx) mode to receive (Rx) mode, capable of avoiding modification ofhardware to transmit a message indicating that a channel is in use, andcapable of saving energy for use in operation of the sensor network anddata transmission/reception.

According to one general aspect, there is provided a method forcollision avoidance in a sensor network in compliance with apredetermined standard, the method including transmitting data by atransmitter; comparing, by the transmitter, a remaining time between thedata transmission and a backoff period with a turnaround time taken forthe transmitter to switch from a transmit (Tx) mode to a receive (Rx)mode; selectively transmitting, by the transmitter, a busy signalindicating that a channel is in use, according to the comparison result;and receiving, by the transmitter, an acknowledgement (ACK) signal froma receiver after the turnaround time has elapsed.

The selectively transmitting of a busy signal may include transmitting,by the transmitter, the busy signal if the remaining time is less thanthe turnaround time.

The selectively transmitting of a busy signal may further includechecking, by the transmitter, channel information thereof using afailure counter indicating a frequency of clear channel assessment (CCA)failure and a congestion control counter indicating whether or not achannel is busy. Further, if the remaining time is less than theturnaround time and the congestion control counter is enabled, thetransmitter may transmit the busy signal.

According to another general aspect, there is provided a method forcollision avoidance in a sensor network in compliance with apredetermined standard, the method including performing clear channelassessment (CCA) by a node rather than a transmitter or a receiver;checking whether or not the node has received a busy signal from thetransmitter; and selectively transmitting, by the node, data to thetransmitter after the turnaround time has elapsed according to the CCAresult and the checking result.

The busy signal may be generated and transmitted by the transmitter if aremaining time between data transmission of the transmitter and abackoff period is compared with a turnaround time taken to switch from atransmit (Tx) mode to a receive (Rx) mode and the remaining time is lessthan the turnaround time. The selectively transmitting of data mayinclude transmitting data from the node to the transmitter only if thechannel is in idle state and the busy signal is not received.

According to another aspect, there is provided a computer-readablerecording medium having embodied therein instructions, which, whenexecuted in a computer system, cause the computer system to perform themethod for collision avoidance in a sensor network.

According to yet another aspect, there is provided an apparatus forcollision avoidance in a sensor network in compliance with apredetermined standard, the apparatus including a communication unitconfigured to transmit and receive data in transmit (Tx) mode andreceive (Rx) mode, respectively; and a control unit configured tocompare a remaining time between the data transmission in the transmitmode and a backoff period with a turnaround time taken to switch fromthe transmit mode to the receive mode, and configured to wait for theremaining time using a delay function. Further, the communication unitmay selectively transmit in the transmit mode a busy signal indicatingthat a channel is in use according to the comparison result of thecontrol unit, and may be switched to the receive mode after theturnaround time has elapsed to receive an acknowledgement signal from areceiver receiving the data.

The communication unit may transmit the busy signal if the remainingtime is less than the turnaround time. The communication unit may checkchannel information thereof using a failure counter indicating afrequency of clear channel assessment (CCA) failure and a congestioncontrol counter indicating whether or not a channel is busy. Thecommunication unit may transmit the busy signal if the remaining time isless than the turnaround time and the congestion control counter isenabled.

Other features will become apparent to those skilled in the art from thefollowing detailed description, which, taken in conjunction with theattached drawings, discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a superframe structure in a slotted IEEE 802.15.4sensor network equipped with collision avoidance mechanism according toan exemplary embodiment of the present invention.

FIG. 2 is a diagram illustrating problems occurring when a remainingtime between data transmission and a backoff period is less than aturnaround time in a sensor network according to an exemplary embodimentof the present invention.

FIG. 3 is a diagram illustrating a process performed when a remainingtime between data transmission and a backoff period is less than aturnaround time according to a method for collision avoidance in asensor network according to an exemplary embodiment of the presentinvention.

FIG. 4 is a flow chart illustrating a method for collision avoidancefrom a point of view of a transmitter in a sensor network according toan exemplary embodiment of the present invention.

FIG. 5 is a flow chart illustrating a process for a transmitter toselectively transmit a busy signal in the method for collision avoidancein FIG. 4.

FIG. 6 illustrates a method for collision avoidance from a point of viewof a third node rather than a transmitter or a receiver in a sensornetwork according to an exemplary embodiment of the present invention.

FIG. 7 is a flow chart illustrating a process of checking channelinformation using a to failure counter and a congestion control counterin a method for collision avoidance in a sensor network according to anexemplary embodiment of the present invention.

FIG. 8 is a block diagram illustrating an apparatus for collisionavoidance in a sensor network according to an exemplary embodiment ofthe present invention.

FIG. 9 is a flow chart illustrating a process for performing operationsin communication is mode from a point of view of a communication unit inthe apparatus for collision avoidance in FIG. 8.

DETAILED DESCRIPTION

Prior to describing exemplary embodiments of the present invention,environments for implementing the exemplary embodiments of the presentinvention will be briefly introduced and basic ideas for the exemplaryembodiments will also be presented. While technologies for implementingIEEE 802.15.4 slotted CSMA/CA (Carrier Sense Multiple Access withCollision Avoidance) are presented, the exemplary embodiments of thepresent invention may be applied to any method and apparatus forcollision avoidance in a sensor network within the scope of thetechnologies.

FIG. 1 illustrates a superframe structure in a slotted IEEE 802.15.4sensor network equipped with a collision avoidance mechanism accordingto an exemplary embodiment of the present invention. Exemplaryembodiments of the present invention will be described using 2.4 GHz(i.e., sixteen channels each having 5 MHz) in the accompanying drawings.

For IEEE 802.15.4 CSMA/CA algorithm, all frames in a superframestructure using beacons use a slotted CSMA/CA algorithm to transmitdata. If the beacons are not used, data transmission is performed usingunslotted CSMA/CA algorithm. In either case, a backoff algorithm isperformed using a backoff period which is a waiting period for datatransmission/reception. The backoff period is defined in twenty (20)symbols. The symbol is the unit of RF data. One byte of data consists oftwo symbols and one symbol has sixteen (16) us; therefore, one byte hasthirty-two (32) us.

For an IEEE 802.15.4 beacon mode, a BI (beacon interval) is divided intoan active part (i.e., SD (superframe duration)) and an inactive part.This interval is adjusted by a coordinator transmitting BO (beaconorder) and SO (superframe order) to neighboring nodes. Beacons providesynchronization to neighboring nodes.

The SD consists of sixteen (16) time slots, which can be further dividedinto the following three parts: beacon, CAP (contention access period)and CFP (contention free period). A GTS (Guaranteed Time Slot) techniqueis employed to implement the CFP. One of the sixteen time slots of theSD is referred to as SlotD (slot duration) which consists of severalbackoff periods each consisting of several symbols. For IEEE 802.15.42.4 GHz hardware, one backoff period has twenty (20) symbols each beingmade up of four bits. The start of the backoff period is referred to asBPS (Backoff Period Start). For a beacon mode, both transmission and CCA(clear channel assessment) are performed at the BPS. For the IEEE802.15.4 standard, when 10 dB or more or eight symbols or more aredetected, CCA recognizes that a channel is busy.

For the IEEE 802.15.4 standard, it is generally determined through theCCA whether or not a communication channel is in an idle state. In thiscase, for the IEEE 802.15.4 slotted CSMA/CA, a coordinator of a sensornetwork sends beacon frames to its neighboring nodes to provide slotsynchronization. Slot synchronization and data transmission will bedescribed in detail.

The first scenario is when a coordinator transmits data to a sensornode. In this case, the coordinator sends the sensor node a beacon framewith information that there is data to transmit. The sensor nodemonitors at regular intervals whether or not beacon information isreceived. If the sensor node receives the beacon information, the sensornode performs time slot synchronization and requests data using a MACcommand frame. In response to this request, the coordinator transmitsthe data to the sensor node. After receiving the data, the sensor nodetransmits an ACK (Acknowledgment) frame to the coordinator.

The second scenario is when a sensor node transmits data to acoordinator. In this case, the sensor node awaits a beacon to receive.If the sensor node recognizes the beacon, synchronization of asuperframe structure is performed. The sensor node transmits data to thecoordinator using the slotted CSMA-CA. The coordinator may optionally anACK frame to the sensor node.

Two scenarios where a remaining time between data transmission and abackoff period is less than a turnaround time will be described withreference to FIG. 2. FIG. 2 is a diagram illustrating problems occurringwhen a remaining time between data transmission and a backoff period isless than a turnaround time in a sensor network according to anexemplary embodiment of the present invention. In FIGS. 2 and 3, Ctx(data transmission coordinator) indicates a coordinator upon datatransmission, Crx (data reception coordinator) indicates a coordinatorupon data reception, Dtx (data transmission device) indicates a sensornode upon data transmission, Drx (data reception device) indicates asensor node upon data reception, and O (Other device) indicates a thirdnode (i.e., a node unrelated to ongoing data transmission/reception)other than a transmitter or a receiver. A time difference between datatransmission and the start of the following backoff period (BPS) isdenoted by t_(r). In FIGS. 2 and 3, a vertical axis indicates the elapseof time, a time axis is divided by the backoff period, and time slots ofelements, such as coordinator, sensor node and third node, aresynchronized with one another.

In FIG. 2, each transmitter transmits data to each receiver and receivesan ACK frame in response. It is assumed that a third node performs CCAto check if a communication channel is in an idle state to allow thethird node to use the communication channel.

If IEEE 802.15.4 beacon-enabled mode and ACK option are set beforehand,the third node performs CCA once or twice according to a CSMA-CAalgorithm. The ACK option is used to indicate whether or not to use anacknowledgement (ACK) signal. The IEEE 802.15.4 standard uses frequencybandwidths of 868 MHz, 915 MHz and 2.4 GHz for operation. The IEEE802.15.4 slotted CSMA-CA in beacon-enabled mode performs the start oftransmission of all kinds of frames and the start of CCA for the backoffperiod. Since the data to transmit varies in size, a difference betweena start point of a backoff period to transmit data and another startpoint of a next backoff period is similarly variable. This variable timedifference is related to a period of time taken for hardware, such as anRF module, to switch from a transmit (Tx) mode to a receive (Rx) mode.The period of time taken for the RF module to switch a mode is referredto as a turnaround time (TAT), which is defined in twelve (12) symbolsin the IEEE 802.15.4 standard.

It is assumed that a node transmitting data is referred to as atransmitter and a node receiving the data is referred to as a receiver.If a remaining time between the time when the transmitter transmits datain a current backoff period and the next backoff period is less thantwelve symbols, the transmitter cannot receive an ACK signal althoughthe receiver sends the ACK signal in the next backoff period sinceswitching of the transmitter from Tx mode to Rx mode needs at least theturnaround time (i.e., a period of time taken for twelve symbols) due toconstrains on the transmitter hardware.

Based on the above, the following equation 1 is established for the IEEE802.15.4 standard:a TurnaroundTime≦t _(ack)≦a TurnaroundTime+a UnitBackoffPeriod   (1)

In the equation 1, the term indicates a period of time between datatransmission and ACK reception, “a TurnaroundTime” indicates aturnaround time (e.g., a period of time taken for twelve symbols), and“a UnitBackoffPeriod” indicates a backoff unit (e.g., a period of timetaken for twenty symbols). Accordingly, for the IEEE 802.15.4 standard,the equation 1 may be expressed in the following equation 2:12≦t_(ack)≦32   (2)

Turning to the equation 1, if a remaining time between data transmissionof the transmitter and a first backoff period is equal to or greaterthan the turnaround time, the transmitter may receive an ACK signal in asecond backoff period. Accordingly, if a third node performs a first CCAin the second backoff period, the third node may recognize that achannel is busy. As a result, the third node does not need to repeat CCAin a third backoff period.

FIG. 2A illustrates remaining time t_(r) between data transmission andthe following backoff period (i.e., BPS (the start of the followingbackoff period)), which is equal to or greater than twelve symbols. Inthis case, after sending data, a transmitter cannot receive an ACKsignal due to hardware constraints before the elapse of a period of timetaken to switch from Tx mode to Rx mode. Accordingly, a remaining timet_(r) of twelve symbols or more means that the transmitter can receivean ACK signal at the immediately following BPS. In the BPS, CCA as wellas data transmission and ACK signal transmission is performed.Accordingly, if a third node performs a first CCA in the following BPS,the third node recognizes that a channel is busy due to the ACK signal,resulting in no additional CCA being performed.

On the other hand, if the remaining time between data transmission ofthe transmitter and the following first backoff period is less than theturnaround time, the transmitter cannot receive an ACK signal in thefollowing second backoff period due to hardware constraints. As aresult, a channel is not in use for the second backoff period and thetransmitter can receive an ACK signal in the following third backoffperiod. In this case, a third node needs to check whether or not achannel is in idle state in the second backoff period to transmit datain the second backoff period. That is, if the third node performs firstCCA in the second backoff period, the third node may recognize that thechannel is in idle state for the second backoff period. However, if thethird node repeats CCA in the third backoff period according to the IEEE802.15.4 standard, the third node recognizes that the channel is busydue to the ACK signal transmitted from the receiver to the transmitter,as described above.

FIG. 2B illustrates remaining time t_(r) between data transmission andthe following backoff period (i.e., BPS (the start of the followingbackoff period)), which is less than twelve symbols. In this case, aftersending data, a transmitter cannot receive an ACK signal in thefollowing BPS due to hardware constraints since a period of time for thetransmitter to switch from Tx mode to Rx mode is not sufficient. In thiscase, the transmitter can receive an ACK signal in the next BPS.Accordingly, if third nodes perform a first CCA in the BPS where the ACKsignal is not transmitted, the third nodes recognize that a channel isin idle state. However, if the third nodes perform a second CCA in thenext BPS where the ACK signal is transmitted, the third nodes canrecognize that the channel is busy due to the ACK signal. That is, thisresults in the unnecessary second CCA being performed to preventcollision.

For the IEEE 802.15.4 standard, the estimated energy consumption of anode is as follows, depending on the states of RF module:

TABLE 1 State Energy (mW) Sleep Power (ESleep) 0.06 CCA Power (ECCA) 51Transmit Power (ETx) 59.1 Receive Power (ERx) 52.2 Idle Power (EI) 2.7

It can be seen from the table 1 that the energy consumption duringperformance of the CCA should be reduced in a sensor network with energyconstraints. The present embodiments are directed to increasingavailable energy for the sensor network by not performing unnecessaryCCA. The sensor network according to the present embodiments isconfigured to save energy by not allowing a third node to perform asecond CCA if it seems apparent that a channel is in idle state in afirst CCA phase but is expected to be used in the next backoff period.This will be described in detail with reference to FIG. 3.

FIG. 3 is a diagram illustrating a process performed when a remainingtime between data transmission and a backoff period is less than aturnaround time according to a method for collision avoidance in asensor network according to an exemplary embodiment of the presentinvention. In FIG. 3, it is assumed that a remaining time t_(r) betweendata transmission of a transmitter and a backoff period is less than aturnaround time consisting of twelve symbols.

As described with reference to FIG. 2, to prevent a third node fromperforming an unnecessary second CCA, the transmitter sends the thirdnode an extra signal informing that a channel corresponding to BPS 310immediately following data transmission of the transmitter is expectedto be used in the next BPS 320. The extra signal is referred to as nBC(notifyBusyChannel). The nBC signal may have, without limitation, aneight-symbol of zeros (0's).

Since the transmitter notifies the third node that a channel is busy dueto the nBC in the BPS 310 immediately following data transmission of thetransmitter, the third node may know that if the third node performs afirst CCA at the same time as the BPS 310, an ACK signal will use thechannel in the next backoff period 320. As a result, the third node doesnot perform an unnecessary second CCA in the next BPS 320. Further,since both a receiver receiving data and existing IEEE 802.15.4 nodesdistributed in the sensor network are not changed in data format, theymay analyze received data according to conventional standards.

The present embodiments will be described in detail with reference tothe accompanying drawings. In the drawings, the same reference numbersdenote the same elements. The present embodiments may be applied to anyapplications employing the IEEE 802.15.4 standard and may also be usedto prevent channel congestion in any applications of wirelesscommunication equipment utilizing CSMA-CA algorithm. Further, in thepresent embodiments, the transmitter is independent of a coordinator ora sensor node.

FIG. 4 is a flow chart illustrating a method for collision avoidance,which is seen from a point of view of a transmitter, in a sensor networkaccording to an exemplary embodiment of the present invention.

In operation 410, a transmitter transmits data to a receiver.

In operation 420, the transmitter compares a remaining time between datatransmission in the operation 410 and a backoff period with a turnaroundtime taken for the transmitter to switch from a transmit (Tx) mode to areceive (Rx) mode.

In operation 430, the transmitter selectively sends a busy signalindicating that a channel is in use, according to the comparison resultof the operation 420. The busy signal is an nBC signal, which isdescribed above. A third node receives the busy signal and uses the busysignal to check whether or not the channel is busy. The selectivetransmission process will be described in detail with reference to FIG.5.

FIG. 5 is a flow chart illustrating a process for the transmitter toselectively transmit the busy signal in the method for collisionavoidance in FIG. 4. A process following the operation 320 where theremaining time t_(r) and the turnaround time are compared to each otherwill be described in detail.

In operation 431, it is determined whether or not the remaining time isless than the turnaround time. If the remaining time is less than theturnaround time, the process proceeds to operation 432. If not, theprocess proceeds to operation 440.

In operation 432, the transmitter checks its channel information using afailure counter indicating the frequency of CCA (clear channelassessment) failure and a congestion control counter indicating if achannel is busy. Such channel information checking has to be performedby the transmitter prior to transmission of an nBC signal. This channelinformation checking will be described in detail with reference to FIG.7.

If a congestion control flag is set in the operation 432, thetransmitter transmits a busy signal (nBC) in operation 433. The nBCsignal is signal data consisting of one to eight symbols of zeros (0's).The nBC signal may vary in size depending on performance of hardwareperforming CCA.

In operation 440, if the remaining time is equal to or greater than theturnaround time, the transmitter waits for the turnaround time andreceives an acknowledgement (ACK) signal from the receiver. In thiscase, data transmission is completed. Since the operation 433 where thetransmitter sends the nBC signal is followed by the operation 440, thetransmitter may receive the ACK signal from the receiver after theturnaround time has elapsed.

Accordingly, the sensor network checks in advance if a channel is busyand sends an nBC signal, thereby preventing unnecessary second CCA frombeing performed. Unless congestion occurs, a channel is used with atypical CSMA-CA technique.

The present embodiment may further include the transmitter checking inadvance a beacon-enabled mode and an ACK option since the second CCA isnot performed if a beacon is not enabled or an ACK signal is not used.Accordingly, the transmitter compares the remaining time with theturnaround time only if both the beacon-enabled mode and the ACK optionare set.

FIG. 6 illustrates a method for collision avoidance, which is seen froma point of view of a third node rather than a transmitter or a receiver,in a sensor network according to an exemplary embodiment of the presentinvention. This method includes operations corresponding to theoperations performed in FIG. 4.

In operation 610, a third node instead of a transmitter or a receiverperforms CCA.

In operation 620, the third node checks whether or not the third nodehas received a busy signal from the transmitter according to the CCA inthe operation 610. The busy signal is a signal that is generated whenthe transmitter compares a remaining time between data transmission ofthe transmitter and a backoff period with a turnaround time taken forthe transmitter to switch from a transmit mode to a receive mode and theremaining time is less than the turnaround time, and is transmitted fromthe transmitter.

In operation 630, it is determined whether or not the busy signal hasbeen received. If the is busy signal has been received, the third nodeperforms no operation since a target channel is in use or is expected tobe used. Unless the busy signal has been received, the process proceedsto operation 640 where the third node sends data to the transmitterafter the turnaround time has elapsed. That is, since the latter casemeans that the target channel is in idle state, the third node may usethe channel.

FIG. 7 is a flow chart illustrating a process of checking channelinformation using a failure counter and a congestion control counter ina method for collision avoidance in a sensor network according to anexemplary embodiment of the present invention. A detailed descriptionwill be mainly made on features which are distinguishable over a typicalCSMA/CA flowchart. In FIG. 7, variables for use in channel accesscontrol are as follows: NB (Number of Backoff) indicates the frequencyof backoff attempts during one frame transmission; BE (Backoff Exponent)indicates a backoff exponent to generate a random backoff period priorto transmission attempt; CW (Contention windows length) indicates thecounter for CCA after the random backoff period. That is, this indicatesthe number of slots to determine whether or not a channel is availableprior to the start of frame transmission. In FIG. 7, CFC (CCA FailureCounter) and CCC (Channel Congestion Control) flags are newly added inthe present embodiment. CFC is a counter to record the frequency of CCAfailure when a node performs CCA, and CCC is used to determine whetheror not channel congestion control is needed. maxCCC is a constant whichis set to be greater than zero (0) by an operator. If CFC is greaterthan maxCCC, the nBC signal is transmitted. Description of unslottedCSMA/CA is omitted to enhance clarity and conciseness.

For the slotted CSMA/CA, a MAC sub-layer initializes NB, CW, CFC and CCCvalues in operation 710 and determines whether or not battery lifeextension is needed in operation 720. Then, the start of a backoffperiod is located. In operation 730, it is delayed as many as the numberof random slots of the backoff period which is determined from a rangefrom 0 to 2^(BE)-1.

In operation 740, the MAC sub-layer requests CCA from a physical layer.If it is determined from the CCA result whether or not a channel is inidle state, it is determined using the CFC and the CCC whether or notthe channel is busy. In the present embodiment, in operations followingCCA, CFC is increased when CCA failure occurs (i.e., when the channel isbusy), while CFC is decreased when CCA is successful (i.e., when thechannel is in idle state). In this case, CFC will not have negativevalues. If CFC has negative values, the congestion control does not haveto be performed since the sensor network is in good condition. If CCAfailure continues to occur, CFC may be increased to infinity. In thiscase, if CFC is increased above a predetermined range, CFC may beinitialized back to zero (0).

In operation 750, CFC is compared with maxCCC which is preset by theoperator. If CFC is greater than the maxCCC, the process proceeds tooperation 760 where CCC flag is set.

In operation 770, if a frame to transmit is a data frame, the remainingtime t_(r) for the data frame is less than twelve symbols, ACK option isset, and CCC is equal to 1, the process proceeds to operation 780 wherethe nBC signal is transmitted. If not, the operation 780 is omitted andthe data frame is transmitted according to a typical IEEE 802.15.4standard. If the data transmission is completed, the process proceeds tooperation 790 where CFC is reduced to a half (CFC/2).

In the algorithm described above, CCA failure rate corresponds tochannel congestion rate. In the present embodiment, CFC used tocalculate the congestion rate in the sensor network is increased one byone and is reduced to a half after the data transmission, therebypreventing a waste of energy due to unnecessary transmission of nBCsignals. In summary, CFC is increased by one if a channel is determinedto be busy from the CCA result, and CFC is reduced to a half if thetransmitter successfully transmits data.

FIG. 8 is a block diagram illustrating an apparatus for collisionavoidance in a sensor network according to an exemplary embodiment ofthe present invention. The apparatus includes a transmitter 10, areceiver 20 and a third node 30. The transmitter 10 includes acommunication unit 11 and a control unit 12. In particular, FIG. 8illustrates data transmission between the transmitter 10, the receiver20 and the third node 30.

The communication unit 11 transmits and receives data in transmit (Tx)mode and receive (Rx) mode, respectively. The communication unit 11transmits data to the receiver 20 and receives an ACK signal from thereceiver 20. The communication unit 11 transmits a busy signal to thethird node 30. It will be apparent to those skilled in the art that thecommunication unit 11 may be implemented by an RF module such as CC2420.Description of the physical structure of the communication unit 11 isomitted to enhance clarity and conciseness.

The control unit 12 compares a remaining time between data transmissionin Tx mode and a backoff period with a turnaround time taken to switchfrom Tx mode to Rx mode, and waits for the remaining time using a delayfunction.

The communication unit 11 selectively transmits a busy signal, whichindicates a channel is busy, to the third node 30 in Tx mode accordingto the comparison result of the control unit 12, and is switched to Rxmode after the elapse of the turnaround time to receive an ACK signalfrom the receiver 20. The communication unit 11 checks channelinformation using a failure counter (i.e., CFC) indicating the frequencyof CCA failure and a congestion control counter indicating if a channelis busy. If the remaining time is less than the turnaround time and thecongestion control counter is enabled, the communication unit 11transmits a busy signal.

The control unit 12 checks beacon-enabled mode and ACK optionbeforehand. If both the beacon-enabled mode and the ACK option are set,the control unit 12 may compare the remaining time and the turnaroundtime and determine whether or not to transmit the busy signal.

It will be apparent to those skilled in the art that the control unit 12may be implemented by an MCU (micro controller unit) such as Atmega128L.

FIG. 9 is a flow chart illustrating a process for performing operationsin communication mode from a point of view of a communication unit inthe apparatus for collision avoidance in FIG. 8. In particular, FIG. 9illustrates RF control of CC2420. Description of a method for collisionavoidance using an nBC signal will be given with reference to FIG. 9.

As described above, since the sensor node to transmit data knows thesize of the data, the sensor node can know whether or not the remainingtime t_(r) is less than twelve symbols. The sensor node can also knowwhether or not the ACK option is preset. Accordingly, in operation 920,CCC determines whether or not to transmit the nBC signal according toresults obtained from congestion control.

If a CFC value is equal to or greater than maxCCC, the process proceedsto operation 930 where it is in GAP HOLD state. Since the GAP HOLD knowsthe size of data, a remaining time (t_(r) symbols) until the next BPS iscalculated, and it is delayed during a corresponding time using a delayfunction.

After the elapse of the delay time, the process proceeds to operation940 where eight symbols of zeros (0's) are transmitted in Tx nBC state.That is, the RF module remains in Tx mode in TX FRAME state of theoperation 910 and is not switched to Rx mode in GAP HOLD state. The RFmodule transmits the nBC signal in TX nBC state and is switched to Rxmode. Through such state change, TAT, which is a period of time forswitching of the RF module from Tx mode to Rx mode, is satisfied.Accordingly, it can be seen from the flow chart of FIG. 9 that thepresent embodiments can be satisfactorily implemented based on hardware.

As apparent from the above description, by selectively transmitting thebusy signal based on comparison results of the remaining time betweenthe data transmission and the backoff period and the turnaround time inthe sensor network according to the present embodiments of theinvention, in a process of switching of communication hardware of sensornodes from Tx mode to Rx mode, other sensor nodes can determine if thechannel is busy without performing unnecessary CCA (clear channelassessment). Furthermore, it is possible to determine if the channel isbusy in both directions of the coordinator and the sensor node using thesame data format without modification of hardware. In addition, it ispossible to save energy for use in operation of the sensor network anddata transmission/reception by reducing the frequency of CCA. Inparticular, since data format needs not to be modified, the sensor nodesaccording to the present embodiments are compatible with other sensornodes of existing sensor networks.

The present invention can be implemented as computer readable code in acomputer readable recording medium. The computer readable recordingmedium includes all types of recording media in which computer readabledata are stored.

Examples of the computer readable recording medium include a ROM, a RAM,a CD-ROM, a magnetic tape, a floppy disk, and an optical data storage.Further, the recording medium may be implemented in the form of acarrier wave such as Internet transmission. In addition, the computerreadable recording medium may be distributed to computer systems over anetwork, in which computer readable code may be stored and executed in adistributed manner. The computer readable code and code segments forimplementing the present invention may be easily inferred by computerprogrammers skilled in the art.

The present invention has been described with reference to the foregoingembodiments. It will be understood by those skilled in the art that theinvention can be implemented in other specific forms without changingthe spirit or essential features of the invention. Therefore, it shouldbe noted that the embodiments are to be considered illustrative in allaspects and are not to be considered in a limiting sense. The scope ofthe invention is defined by the appended claims rather than the detaileddescription of the invention. All changes which come within the range ofequivalency of the claims should be construed as falling within thescope of the invention.

What is claimed is:
 1. A method for collision avoidance in a sensornetwork in compliance with a predetermined standard, the methodcomprising: transmitting data by a transmitter; comparing, by thetransmitter, a remaining time between the data transmission and abackoff period with a turnaround time taken for the transmitter toswitch from a transmit (Tx) mode to a receive (Rx) mode; selectivelytransmitting, by the transmitter, a busy signal indicating that achannel is in use, according to the comparison result; and receiving, bythe transmitter, an acknowledgement (ACK) signal from a receiver afterthe turnaround time has elapsed, wherein the selectively transmitting ofa busy signal comprises transmitting, by the transmitter, the busysignal if the remaining time is less than the turnaround time.
 2. Themethod of claim 1, wherein the selectively transmitting of a busy signalfurther comprises checking, by the transmitter, channel informationthereof using a failure counter indicating a frequency of clear channelassessment (CCA) failure and a congestion control counter indicatingwhether or not a channel is busy, and wherein if the remaining time isless than the turnaround time and the congestion control counter isenabled, the transmitter transmits the busy signal.
 3. The method ofclaim 2, wherein the failure counter is increased by one if a channel isdetermined to be busy as a result of the CCA, and the failure counter isreduced to a half if the transmitter successfully transmits the data. 4.The method of claim 1, wherein the selectively transmitting of a busysignal comprises the transmitter waiting for the turnaround time if theremaining time is equal to or greater than the turnaround time.
 5. Themethod of claim 1, further comprising checking, by the transmitter, abeacon-enabled mode and an ACK option beforehand, wherein the remainingtime is compared with the turnaround time if the beacon-enabled mode andthe ACK option are set.
 6. The method of claim 1, wherein thetransmitter is at least one of a coordinator and a sensor node.
 7. Themethod of claim 1, wherein the predetermined standard is a slotted IEEE802.15.4 standard.
 8. A method for collision avoidance in a sensornetwork in compliance with a predetermined standard, the methodcomprising: performing clear channel assessment (CCA) by a node ratherthan a transmitter or a receiver; checking whether or not the node hasreceived a busy signal from the transmitter; and selectivelytransmitting, by the node, data to the transmitter after the turnaroundtime has elapsed according to the CCA result and the checking result,wherein the busy signal is generated and transmitted by the transmitterif a remaining time between data transmission of the transmitter and abackoff period is compared with a turnaround time taken to switch from atransmit (Tx) mode to a receive (Rx) mode and the remaining time is lessthan the turnaround time.
 9. The method of claim 8, wherein theselectively transmitting of data comprises transmitting data from thenode to the transmitter only if the channel is in idle state and thebusy signal is not received.
 10. The method of claim 8, wherein thepredetermined standard is a slotted IEEE 802.15.4 standard.
 11. Anon-transitory computer-readable recording medium having embodiedtherein instructions, which, when executed in a computer system, causethe computer system to perform the method of claim
 1. 12. Anon-transitory computer-readable recording medium having embodiedtherein instructions, which, when executed in a computer system, causethe computer system to perform the method of claim
 8. 13. An apparatusfor collision avoidance in a sensor network in compliance with apredetermined standard, the apparatus comprising: a communication unitconfigured to transmit and receive data in transmit (Tx) mode andreceive (Rx) mode, respectively; and a control unit configured tocompare a remaining time between the data transmission in the transmitmode and a backoff period with a turnaround time taken to switch fromthe transmit mode to the receive mode, and configured to wait for theremaining time using a delay function, wherein the communication unitselectively transmits in the transmit mode a busy signal indicating thata channel is in use according to the comparison result of the controlunit, and is switched to the receive mode after the turnaround time haselapsed to receive an acknowledgement signal from a receiver receivingthe data, wherein the communication unit transmits the busy signal ifthe remaining time is less than the turnaround time.
 14. The apparatusof claim 13, wherein the communication unit checks channel informationthereof using a failure counter indicating a frequency of clear channelassessment (CCA) failure and a congestion control counter indicatingwhether or not a channel is busy, and the communication unit transmitsthe busy signal if the remaining time is less than the turnaround timeand the congestion control counter is enabled.
 15. The apparatus ofclaim 13, wherein the control unit checks a beacon-enabled mode and anACK option beforehand, and wherein if the beacon-enabled mode and theACK option are set, the control unit compares the remaining time withthe turnaround time to determine whether or not to transmit the busysignal.
 16. The apparatus of claim 13, wherein the predeterminedstandard is a slotted IEEE 802.15.4 standard.